Ricardo Gracia Ballarín

Effects of tillage on the soil water retention curve during a fallow period of a semiarid dryland

Tillage practices have a significant influence on the soil hydro-physical properties. The objective of this work was to evaluate the effect of tillage on the α (a scaling factor) and n (a pore size distribution parameter) van Genuchten soil water retention curve parameters during an 18-month long fallow period in a semiarid dryland. Three different tillage systems employed during 23 years of trials were compared: conventional (CT), reduced (RT) and no-tillage (NT). Measurements of soil bulk density (ρb) and the soil water retention curve θ(ψ) were performed at 0–10, 10–20 and 20–30cm soil depths. The θ(ψ) was determined with the Time Domain Reflectometry (TDR)-pressure cells at the following pressure heads: 0.5, 1.5, 3, 10, 50, 100, 500 and 1500kPa. From these data, α, n and the SDexter index were evaluated. The 0–40cm depth soil volumetric water content, θ, was also measured in the field using the TDR technique. Compared with CT and RT, NT had the highest θ values during all the fallow period. No significant influence of soil depth on θ(ψ) was observed in all tillage treatments at each sampling date. Although under consolidated soil conditions no significant differences in ρb and the water content at saturation (θs) were observed among tillage treatments, NT had the highest and lowest values of ρb and θs, respectively. The loosening of soil due to tillage practices in CT and RT significantly decreased ρb and increased θ at the wet-end section of θ(ψ). Post-tillage rainfall resulted in significant decreases in θs, α and the maximum value of the pore size distribution (PSDmax). The different soil structure created by mouldboard ploughing (CT) and chiselling (RT) explained the higher PSDmax under RT than CT. The most important changes in θ(ψ) followed the first copious effective rainfall events (>10mm) after tillage. These facts enabled the soil to recover the pre-tillage water retention curve shapes and the van Genuchten parameters pre-tillage values. Effective rainfall events in the late fallow had a minor effect on the water retention curve. Although tillage tended to increase n, this change was not significant. The SDexter index, which was also affected by tillage, was greater than 0.035 during all the fallow period, indicating good soil physical quality.